A. Field of the Invention
This invention relates to a continuous process for depositing a metal at the cathode of an electrolytic cell. More particularly, the invention is concerned with a continuous cyclic technique wherein (a) Fe.sup.+2 ion is oxidized to Fe.sup.+3 in an aqueous acidic electrolyte at the anode of an electrolytic cell with the corresponding electrodeposition of a metal at the cathode, and (b) the reduction of Fe.sup.+3 generated at the anode with a solid carbonaceous reductant material to Fe.sup.+2 for subsequent reuse in the process.
B. Prior Art
It is well known that carbon and carbonaceous materials may be oxidized at the anode in aqueous electrolyte in an electrochemical cell through which a direct current flows.
Recently, a renewed interest in the electrochemical oxidation of carbonaceous materials has developed wherein coal-assisted generation of hydrogen, or deposition of metals, has been proposed. Thus, U.S. Pat. No. 4,268,363 teaches the electrochemical gasification of carbonaceous materials by anodic oxidation which produces oxides of carbon at the anode and hydrogen or metallic elements at the cathode of an electrolysis cell.
U.S. Pat. No. 4,226,683 teaches the method of producing hydrogen by reacting coal or carbon dust with hot water retained as water by superatmospheric pressure. The pressure is controlled by the use of an inert dielectric liquid which washes the electrodes and while doing so depolarizes them by absorption of the gases.
U.S. Pat. No. 4,233,132 teaches a method wherein the electrodes are immersed within oil which forms a layer over a quantity of water. When current is passed between the electrodes, water is caused to undergo electro-decomposition. Gaseous hydrogen is collected in the sealed space above the oil-water layers, and the oxygen is believed to react with the constituents in the oil layer.
These represent some of the prior art in attempting to produce useful rates of electrochemically assisted oxidation of carbonaceous fuels. A further example is the use of carbonaceous fuels at the anode of a fuel cell, such devices having failed to achieve comxercial realization due to the products of combustion reducing the efficiency of the system, tars forming on the catalytic surfaces, and the poisoning effect of sulfur and CO.
As acknowledged in U.S. Pat. No. 4,226,683, the principal problem in the past use of this technology was the slow rate of the electrochemical reaction of coal or carbon and water at the anode.
U.S. Pat. No. 3,761,369 teaches the cathodic electrodeposition of copper accompanied by anodic oxidation of Fe.sup.+2 to Fe.sup.+3.
U.S. Pat. No. 4,202,744 teaches a method wherein elemental iron is oxidized in an aqueous solution of an alkali metal hydroxide at the anode of an electrolytic cell with simultaneous generation of hydrogen at the cathode. The iron oxidation products of the reaction are thereafter reduced to elemental iron by contact with a carbonaceous reducing agent at elevated temperatures and the reduced material recycled for reoxidation. Carbon monoxide is the preferred reducing agent and temperatures above 1000.degree. F. are recommended.
Fray et al, British Patent Application No. 2,087,431A and U.S. Pat. No. 4,412,893, disclose that iron (III) ions generated at the anode of an electrochemical cell may be reduced to iron (II) ions by contacting the iron (III) ions with lignite at a temperature greater than 40.degree. C. in a vessel external to the cell.
U.S. Pat. No. 4,405,420 of common inventive entity and assignee to this application, discloses that iron, when added to an electrolyte containing carbonaceous material at the anode, and preferably iron in the +2 and +3 valence state, catalyzes the rate of reaction significantly, in some instances higher than two orders of magnitude over the uncatalyzed system with concurrent electrodeposition of a metal at the cathode, which application is incorporated by reference.
A process whereby an aqueous acidic flow of iron (II) is oxidized to iron (III) at the anode of an electrochemical cell and then cycled to a carbonaceous bed wherein it is reduced to iron (II) in a continuous manner significantly enhances the commercial feasibility of the process. Such a continuous process would necessarily require that the carbonaceous material and operating conditions be of such a nature as to allow for sustained oxidation of the carbonaceous material since an unsustained oxidation of the carbonaceous material would require constant interruption of the flow in order to replenish the carbonaceous reductant material.
As used herein, the terms "sustained oxidative reactivity", "sustained oxidation of the carbonaceous material" and the like, means that the oxidation rate of the carbonaceous material does not exhibit significant decay due to the inability of the iron (II) ions to penetrate the oxidized surface of the carbonaceous material. For the purpose of this definition, the rate of oxidation of the carbonaceous material with iron (III) ions may be expressed as the rate of formation of iron (II). The overall reaction order for this process is believed to be ##EQU1## wherein Csurf represents available non-oxidized carbon surface on the carbonaceous material. Sustained rates for the purpose of this invention are those wherein the reaction rate, as defined above, is maintained at least at 1.times.10.sup.5 Mol.sup.-1 Min.sup.-1 for a period of at least 5 hours.
It has now been found that in order to sustain such a reaction rate for the oxidation of the carbonaceous material in a continuous process, the following criticalities must be met:
1. The surface area of the carbonaceous material must be substantially free of carboxylic or carbonyl groups in order to allow penetration of the iron (III) ions onto the non-oxidized carbon surface thus allowing the generation of iron (II) ions. Generally, carbonaceous materials which are substantially free of carboxylic or carbonyl groups are those which contain less than 30% oxygen as carboxylic or carbonyl groups. Accordingly, carbonaceous materials which contain greater than 30% oxygen as carboxylic or carbonyl groups (lignite) are not suitable for this invention.
2. Temperatures of 120.degree. C. and greater.
Temperatures of 120.degree. C. and greater are particularly surprising in view of Farooque et al, Fuel, 58, 705-7515, October 1978, where it was stated that it would be possible to consume coal to a much larger extent at a meaningful rate by conducting the electrochemical gasification at temperatures of 200.degree. C. and greater.
After oxidation of the carbonaceous material to about 30% oxygen (as carboxylic or carbonyl groups), the rate of reaction slows becoming more dependent upon the decomposition of the oxidized carbonaceous material--that is upon the rate of decarboxylation (CO.sub.2 elimination) from the carbonaceous material. The rate of decarboxylation is slow and does not approach a sustained rate until temperatures of about 270.degree. C. and greater are employed. Accordingly, at 270.degree. C. and above, carbonaceous materials containing greater than 30% oxygen as carboxylic or carbonyl groups will sustain the oxidative process in a continuous process since carboxylic groups are readily eliminated from the carbonaceous surface as CO.sub.2.